EP 0 948 241 A2 discloses a circuit arrangement for operating light emitting diodes, which has an input for inputting an input voltage and an output for outputting to the light emitting diodes. In the case of the circuit disclosed therein, the series-connected LEDs are connected in series with the inductor N1, which is in turn connected in series with a switch K1, and are connected to the voltage supply. The switch K1 is opened when a predetermined upper threshold value, i.e. a predetermined switch current, is reached. This mode of operation is known to the person skilled in the art as current mode control, on the basis of the signal of the shunt R2. In the subsequent demagnetization phase, the inductor current freewheels via the diode connected back-to-back with respect to the light emitting diodes and the inductor. If the freewheeling current reaches a predetermined lower threshold value, the switch K1 is closed again and the inductor is magnetized anew. One prerequisite for the function described is that the input voltage Uin is always greater than the forward voltage of the light emitting diodes.
In EP 0 948 241 A2, the inductor N1 is embodied as a winding of a transformer, with the result that an auxiliary voltage supply can be realized by means of the winding N2 and also D2 and C2. The circuit is started up via the R1 directly by the input voltage Uin. The auxiliary winding N2 has a further task: via said auxiliary winding, the freewheeling current is measured indirectly by means of the circuit part C, which supplies a control signal for switching the switch K1 on again. If the inductor is demagnetized, the voltage at the winding N2 jumps, which is detected by the circuit part C. The transformer can be embodied as a three-winding transformer, wherein the third winding N3 together with the circuit part B realizes an additional synchronous rectification with respect to the diode D1.
The circuit arrangement has the major disadvantage, however, that the switch K1 is generally subjected to hard switching, that is to say that ZVS (zero voltage switching) is not implemented; in the case of ZVS, the circuit is operated in such a way that the corresponding switch is switched whenever the voltage across the switch is substantially zero. This is not the case in the circuit arrangement according to EP 0 948 241 A2; in particular in the case of a non-intermittent, i.e. constant, current through the light emitting diodes, the reverse recovery effect of the diode D1 leads to a significant reduction of the efficiency of the circuit, which, in particular in the case of a rising switching frequency—necessary for miniaturization—leads to a decreasing efficiency owing to rising switching losses.
The article “Zero Voltage Switching Resonant Power Conversion”, printed in the seminar manual “Switching Regulated Power Supply Design” from Unitrode Corporation, published in 1990, discloses a circuit arrangement in accordance with FIG. 2, having an input for inputting an input voltage and an output for outputting an output voltage to a load. This circuit arrangement operates with ZVS; consequently, the switching losses are minimized. If one or a plurality of series-connected light emitting diodes are connected to this circuit arrangement, then said diodes are fundamentally operated in pulsed fashion since a pulsating DC voltage is applied to the load, and, contrary to the illustration in FIG. 2 of the article, the load does not approximately behave like a current source (designated as IOUT in the article). The light emitting diodes are turned on in one half-cycle; the diode D0 is turned on in the other half-cycle. However, the pulsed mode of operation is not optimum for a good efficiency of the light emitting diodes. Moreover, the optical appearance of the light emission can be impaired in the case of pulsed operation.